When you bought the new vacuum unit did you test it before you installed it? Can you use the hand vacuum pump to make the vacuum unit work on the front axle? Can you connect the hand vacuum unit to the line on the truck to read the vacuum available on the truck when it is running and in 4x4? Just some of my first ideas before tearing into the axle.
Bryce

Every electronic device is manufactured with smoke stored deep inside... only a true genius can find a way to set it free. The most terrifying words in the English language are: "I'm from the government and I am here to help!"